Treating hot water process discharge water by flocculation and vacuum precoat filtration



March 24, 1970 P. s. HEPP ET AL 3,502,575

TREATING HOT WATER PROCESS DISCHARGE WATER BY FLOCCULATION AND VACUUMPRECOAT FILTRATION Filed June 20, 1968 DILUENT CENTRIFUGE 22 ZONE 24BITUMEN TAR SANDS PRODUCT COMBINED FROTH\ 23 WATER SETTLED SCAVENGERFROTH FROTH CONDITIONING PRIMARY DRUM FROTH SETTLER 2o SCAVENGER FROTHSCREEN s \SOVERSIZE WATER |6\ m SEPARATION w ZON --l5 L J4 A|R FLOTATIONSCAVENGER 4 ZONE TAILINGS 26 2? 26 SAND 4| PUMP 29 SAND DISTRIBUTIONZONE ZONE P|P|NG POND ZONE 2 INVENTOR.

PETER S. HEPP FREDERICK W. CAMP BY EMLM ATTORNE United States Fatent OABSTRACT OF THE DISCLOSURE The specification discloses a process forclarifying water discharged from a hot water process for separatingbitumen from tar sands. This Water is subjected to flocculation of itsclay component followed by vacuum precoat filtration to produce aclarified Water suitable for recycle as at least a portion of the feedwater to the hot water process.

This invention relates to a process for clarifying water discharged froma hot water process for separating bitumen from tar sands. It has beenfound that flocculation and vacuum precoat filtration can be applied toclay and silt containing discharge to produce a clarified water suitablefor recycle as at least a portion of the feed water to the hot waterprocess.

Numerous deposits of bituminous tar sands exist throughout the world.The most extensive deposits are found in northern Alberta, Canada. Thesands are composed of a siliceous material, generally having a sizegreater than that passing a 325 mesh screen, saturated with a relativelyheavy, viscous bitumen in quantities of from 5 to 21 Weight percent ofthe total composition. More typically the bitumen content of the sand-sis between about 8 to 15 percent. This bitumen is quite viscous andcontains typically 4.5 percent sulfur and 38 percent aromatics. Itsspecific gravity at 60 F. ranges typically from about 1.00 to about1.06. The tar sands also contain clay and silt. Silt is defined asmineral which will pass a 325 mesh screen but which is larger than 2microns. Clay is mineral smaller than 2 microns including some siliceousmaterial of that size.

There are several well-known processes for effecting separation ofbitumen from the tar sands. In the hot water method, the bituminoussands are jetted with steam and mulled witha minor amount of hot waterat temperatures in the range of 140 to 210 F. The resulting pulp isconducted to a sump where it is diluted with additional hot water andcarried to a separation cell maintained at a temperature of about 150 to200 F. In the separation cell,

sand settles to the bottom as tailings and bitumen rises to the top inthe form of an oil froth. An aqueous middlings layer containing somemineral and bitumen is formed between these layers. A scavengers stepmay be conducted on the middlings layer from the primary separation stepto recover additional amounts of bitumen therefrom. This step usuallycomprises aerating the middlings as taught by K. A. Clark, The Hot WaterWashing Method, Canadian Oil and Gas Industries 3, 46 (1950). Thesefroths can be combined, diluted with naphtha and centrifuged to removemore water and residual mineral. The naphtha is then distilled off andthe bitumen is coked to a high quality crude suitable for furtherprocessing.

The hot water process is described in detail in Floyd et al., UnitedStates application Ser. No. 509,589. Floyd et al. disclose that frothformation in the process separation cell is dependent upon the viscosityof the middlings layer, and that the viscosity is dependent upon themiddlings clay 3,502,575 Patented Mar. 24, 1970 content. It is thoughtthat increasing viscosity retards the upward settling of bitumen flecks.When this occurs, the smaller bitumen flecks and those that are heavilyladen with mineral matter stay suspended in the water of the cell andare removed with the middlings layer, or are lost altogether With thesand failings.

Upon discharge from the separation zone the Water from the middlingsmust eventually be stored, disposed of, or recycled back into theprocess. Because this water contains bitumen emulsions, finely dispersedclay With poor settling characteristics and other contaminants, waterpollution considerations prohibit discarding the water into rivers,lakes or other natural bodies of water. It has been proposed that thewater be stored in evaporation ponds but this proposal would involvelarge space requirements and the construction of expensive enclosuredikes. It has also been suggested that the Water in the efiluentdischarge be recycled back into the process as an economic measure toconserve both heat and water. Floyd et al. teach that some of this Watercan be so recycled but that the amount of recycle is limited by thedispersed silt and clay content of the water which can reduce frothyield by increasing the viscosity of the middlings layer and retardingthe upward settling of bitumen flecks as pointed out supra. A proportionof water in the diluted tar sands pulp fed into the separation cell musttherefore be fresh water-water which is substantially free of the clayand silt found in middlings water. In fact, with some high. clay contenttar sands feeds, all of the water in the diluted pulp must be added asfresh water.

It has been found that silt and clay containing water discharged fromthe process can be made suitable for recycle as at least a portion ofthe hot water process water feed by treating the discharge according toa method of flocculation and precoat vacuum filtration described herein.It is known that flocculation, even with extended settling, cannot beused to clarify water from a hot water process. Furthermore, it has beenfound that precoat vacuum filtration alone cannot be used to clarify thewater. It has been found by the present invention that precoat vacuumfiltration of a water discharge in which the clay component has beenflocculated does produce a clear filtrate suitable for further use ordiscard. Rotary precoat filtration procedures typically compriserotating a cylindrical filter drum or chamber to continuously submerge asegment or portion of the peripheral screen filtering surface, carryinga vacuumor pressure-held filter medium consisting essentially of apermeable bed or cake of filter aid, into the suspension or slurry ofliquid and solids to be filtered, the pressure differential inducingpassage of the liquid filtrate component of said suspension or slurrythrough the filtering medium, thereby separating the liquid from thesolids, the solids being retained on the surface of, or entrappedwithin, the interstices of the precoat cake adjacent to the surface.Collection and retention of thesolids on the surface and within theinterstices of the filter medium adjacent the surface, however, fill andthus block the pores or interstices, inhibiting further passage andseparation of filtrate which necessitates the removal of the surfaceportion of the precoat containing the retained and entrapped solids topermit further filtration. In rotary precoat filtration proceduresremoval of retained solids is typically effected by means of acontinuously advancing doctor knife or blade which penetrates into theprecoat cake to a depth approximately equal to that reached by theentrapped solids. Thus, uninterrupted filtration through rotation of thecylindrical filter drum is permitted by the continuous cutting away andremoving of the retained solids with their entrapping precoat cake andin turn exposing a substantially uncontaminated new surface for furtherfiltration.

The process of the present invention is an improvement to the hot waterprocess for separating bitumen from tar sands. The improvement comprisessubjecting at least a portion of the effiuent discharge from the hotwater process to clarification by flocculation followed by vacuumprecoat filtering. It is quite surprising that flocculation and vacuumprecoat filtering substantially clarify the water portion of thiseffluent in view of its unique composition, i.e., water containing about0.1 to 1.5 weight percent bitumen and up to about 20 weight percentmineral between 80 and 100 percent of which is fine clay of a sizesmaller than 2 microns. A substantial portion (about 50 percent byweight) of this mineral is fines of a size smaller than 0.2 micron. Thisfine clay has extremely poor settling characteristics. It is thismaterial in combination with other components of the water which makesthis water portion diflicult to clarify to a degree such that the wateris suitable for reuse or discard.

The present invention achieves clarification of this water byflocculation and application of a vacuum precoat filtration techniquewhereby at least a portion of the water discharged from the hot waterprocess is subjected to a flocculation process and is passed to a vacuumprecoat filter zone. This zone comprises a rotary filter drum theperiphery of which is covered with a filtering medium, means formaintaining a vacuum in the drum, a feed vessel for containing the waterportion through which the filtering surface of the drum moves as thedrum rotates, a blade located in shaving relationship to the drum outersurface for maintaining the filtering medium layer on the drum at adecreasing depth, and a receiving vessel for receiving under vacuum theclear filtrate recovered from the drum. The drum is rotated so that itsfiltering surface moves through the water discharge portion held in thefeed vessel. A substantially clarified water is withdrawn by vacuum fromthe discharge portion into the drum and thence into the filtratereceiver, and the filtering medium is coated with mineral from thedischarge. The coating mineral is shaved from the filtering medium bythe blade. The substantially clarified water which is recovered can berecycled as at least a portion of the water used to form the mixture ofbituminous tar sands and water which is passed to the separation zone inthe hot water process.

The invention can be described in more detail with reference to thefigure which is a schematic representation of the present improvement toa hot water process.

In the figure,-bituminous tar sands are fed into the system through line1 where they first pass to a conditioning drum or muller 3. Water andsteam are introduced from 2 and mixed with the sands. The total water sointroduced is a minor amount based on the weight of the tar sandsprocessed and generally is in the range of 10 to 45 percent by weight ofthe total mixture. Enough steam is introduced to raise the temperaturein the conditioning drum to within the range of 130 to 210 F. andpreferably to above 170 F. Monovalent alkaline reagents can also beadded to the conditioning drum, usually in amount of from 0.1 to 3.0pounds per ton of tar sand. The amount of such alkaline reagentpreferably is regulated to maintain the pH of the middlings layer inseparator zone 12 within the range of 7.5 to 9.0. Best results areobtained at a pH value of 8.0 to 8.5. The amount of the alkaline reagentthat needs to be added to maintain a pH value in the range of 7.5 to 9.0can vary from time to time as the composition of the tar sands asobtained from the mine site varies. The best alkaline reagents to usefor this purpose are caustic soda, sodium carbonate or sodium silicate,although any of the other monovalent alkaline reagents can be used ifdesired.

Mulling of the tar sands produces a pulp which then passes from theconditioning drum as indicated by line 4 to a screen indicated at 5. Thepurpose of screen 5 is to remove from the tar sand pulp any debris,rocks or oversized lumps as indicated generally at 6. The pulp thenpasses from screen 5 as indicated by 7 to a sump 8 where it is dilutedwith additional water from 9 and a middlings recycle stream 10 In theevent the clay content of the tar sands is high, a relatively high rateof fresh or treated feed water introduction through 9 can be employed tocompensate for the high clay introduc tion while a correspondingly highrate of transfer of middlings layer through line 15 as hereinafterdescribed can be maintained. Under these circumstances recycling of theother stream of middlings through line 10 to the sump is not required.

Modifications that may be made in the process as above described includesending a minor portion of the middlings recycle stream from line 10through a suitable line (not shown) to the conditioning drum 3 to supplyall or a part of the water needed therein other than that suppliedthrough condensation of the stream which is consumed. Also, if desired,a stream of the middlings recycle can be introduced onto the screen 5 toflush the pulp therethrough and into the sump. As a general rule thetotal amount of water added to the natural bituminous sands as liquidwater and as steam prior to the separation step should be in the rangeof 0.2 to 3.0 tons per ton of the bituminous sands. The amount of waterneeded within this range increases as the slit and clay content of thebituminous sand in creases. The amount of water added should beregulated along with the amount of middlings withdrawn via line 15 so asto maintain the viscosity of the separation zone middlings between about0.4 to 5.7 centipoises, preferably about 1 to 2 centipoises. As ageneral rule the rate of withdrawal of bitumen-rich middlings toscavenger zone 16 will be 10 to 75 gallons per ton of tar sandsprocessed when 15 percent by weight of the mineral matter is below 44microns and to 250 gallons per ton when from 25 to 30 percent of themineral is of this fine particle size. The amount of water added willgenerally be about 0.3 to 0.5 ton per ton of tar sands when 15 percentby weight of mineral matter of the bituminous sands has a particle sizebelow 44 microns. On the other hand, when 30 percent of the mineralmatter is below 44 microns diameter, generally 0.7 to 1.0 ton of watershould be used per ton of tar sand.

Further following the process, the pulped and diluted tar sands arepumped from the sump through line 11 into the separation zone 12. Thiszone comprises a cell which contains a relatively quiescent body of hotwater. In the cell, the diluted pulp forms into a bitumen froth layerwhich rises to the cell top and is withdrawn via line 13, and a sandtailings layer which settles to the bottom to be withdrawn through line14. An aqueous middlings layer between the froth and tailings layercontains silt and clay and some bitumen which failed to form froth. Inorder to prevent the buildup of clay in the system and maintain themiddlings viscosity within the desired range, it is necessary tocontinually remove some of the middlings layer and supply enough waterin the conditioning operations to compensate for that so removed. Therate at which the middlings need to be removed from the system dependsupon the content of clay and silt present in the tar sands feed and thiswill vary from time to time as the content of these fines varies. If theclay and silt content is allowed to build up in the system, theviscosity of the middlings layer will increase. Concurrently with suchincrease, an increase in the proportions of both the bitumen and thesand retained by the middlings will occur. If the clay and silt contentis allowed to build up too high in the system, effective separation nolonger will occur and the process will become inoperative. This can beavoided by regulating the recycling and withdrawal of middlings andinput of fresh water per the invention disclosed and claimed in theFloyd et al. application. However, even when the separation step isoperating properly the middlings layer withdrawn through line 15 willcontain a substantial amount of bitumen which did not separate. Hencethe middlings layer withdrawn through line is, for purpose ofdescription, herein referred to as oil-rich or bitumen-rich middlings.

The oil-rich middlings stream withdrawn from separator 12 through line15 is sent to a scavenger zone 16 wherein an air flotation operation isconducted to cause the formation of additional bitumen froth. Theprocessing conducted in the scavenger zone 16 involves air flotation byany of the air flotation procedures conventionally utilized inprocessing of ores. This involves providing a controlled zone ofaeration in the flotation cell at a locus where agitation of themiddlings is being effected so that air becomes dispersed in themiddlings in the form of small bubbles. Aeration causes the formation ofadditional bitumen froth which passes from the scavenger zone 16 throughline 17 to a froth settler zone 18. A bitumen-lean middlings stream isremoved from the bottom of the scavenger zone 16 via line 19.

In the settler zone 18, the scavenger froth forms into a lower layer ofsettler tailings which is withdrawn and recycled via line 20 to be mixedwith bitumen-rich middlings for feed to the scavenger zone 16 via line15. In the settler zone an upper layer of upgraded bitumen froth formsabove the tailings and is withdrawn through line 21 and is mixed withprimary froth in line 13. The combined froths are at a temperature ofabout 160 F. They are heated with steam and diluted with sufficientnaphtha or other diluent from 22 to reduce the viscosity of the bitumenfor centrifuging in zone 23 to produce a bitumen product 24 suitable forfurther processing.

The oil-lean middlings in line 19 and the sand tailings in line 14 arecombined to form an efiiuent discharge which is delivered via line 25 toa sand pile zone 26 via distribution piping 27. The effluent containsbetween 25 and 50 weight percent sand and silt material which is largerthan about two microns. The distribution piping provides for continuousand uniform delivery of the efliuent to the sand pile zones where thesand and silt material is deposited. The water in the eflluent dischargepercolates down through and over the sand pile zone 26 to the pond zone28 where it collects as pondwater containing up to about 20 weightpercent suspended solids, between 80 and 100 percent of which is fineclay of a size smaller than two microns. The pondwater also containsbetween about 0.1 and 1.5 weight percent bitumen. Because of theparticular composition of this pondwater, and especially because of theextreme fineness of the suspended clay material which has extremely poorsettling characteristics, the water cannot be discarded or to any greatextent recycled back into the hot water system.

By the improvement of the present invention, the pondwater is withdrawnfrom pond zone 28 by means of pump 29 and fed via 30 into zone 31 Wheresuspended solids in the water are flocculated. For the purpose of thisinvention, pondwater is effluent discharge from a hot water processwhich elfluent has been settled to give a composition comprising watercontaining up to about 20 percent solids, between 80 percent and 100percent of which is fine clay of a size smaller than 2 microns. Theeflluent discharge from a hot water process comprises middlings materialof depleted bitumen content which has undergone final treatment, thesand tailings layer from the process and other dischargedwater-containing fractions which are not the primary products of the hotwater process. The discharge is removed from the process area as aslurry of about 25 to 60, typically 45, percent solids by weight. Theefiluent contains virtually all of the clay material which was presentin the feed. Typically the amount is 2 to 10 weight percent of the feed.This material is smaller than two microns and has extremely poorsettling characteristics.

The flocculation step on the pondwater can be carried out by adding aconventional flocculating reagent via line 44 to the Water with gentleagitation. Among the various reagents useful for flocculating clay arealuminum sulfate (alum), polyalkylene oxides such as polyethylene oxide,compounds of calcium such as calcium hydroxide, calcium oxide, calciumchloride, calcium nitrate, calcium acid phosphate, calcium sulfate,calcium tartrate, calcium citrate, calcium sulfonate, calcium lactate,the calcium salt of ethylene diamine tetraacetate and similar organicsequestering agents. Also suitable are guar flour or a high molecularweight acrylamide polymer such as polyacrylamide or a copolymer ofacrylamide and a copolymerizable carboxylic acid such as acrylic acid.Additional flocculants include the polymers of acrylic or methacrylicacid derivatives, for example, acrylic acid, methacrylic acid, thealkali metal and ammonium salts of acrylic acid or methacrylic acid,acrylamide, methacrylamide, the aminoalkyl acrylates, the aminoalkylacrylamides, the aminoalkyl methac rylamides and the N-alkyl substitutedaminoalkyl esters of either acrylic or methacrylic acids.

Preferably flocculation is accomplished by changing the pH of the water.Classically clay is flocculated by catins such as iron and aluminum bythe mechanism of compression of the double layer charge on the suspendedparticles. Variation of pH causes a change in the charge on the edges ofthe clay particles, thus allowing flocculation. The effiuent dischargefrom the hot water process has a pH ranging from about 7.5 to 9.0,typically about 8.3. In this range, the contained mineral material doesnot fiocculate; however, raising the pH above about 9.0 or lowering itbelow about 7.5 does cause flocculation. More preferably flocculation inzone 31 is accomplished by adding sulfuric acid (to the pondwater vialine 44 or the middlings or eflluent discharge portion as the case maybe) to about pH 5.

If desired a filter aid can be added in zone 31 via line 45 -to aid inthe subsequent vacuum precoat flltration step.

Such aids are solids added to filter feeds for filtering out with thefeed solids. The purpose of the aid is to increase filter cake porositythereby increasing filtration rate. Commonly the same material used asprecoat is also used as a filter aid; however, only filter aids of fineparticles size are effective in the present invention. An example ofsuch an effective aid is a fine diatomaceous silica with an averageparticle diameter of about 4 microns. It has been found thatconventional aids of average particle diameters greater than about 6microns are ineffective for increasing filtration rate in the process ofthe present invention.

Further with reference to the drawing, water containing flocculated claysolids and with or without a filter aid is passed from zone 31 via 32 toa feed vessel 33 in a precoat vacuum filtration zone which is indicatedin the drawing as the combination of the vessel 33, a rotary filter drum34, a shaving blade 35 and a filtration receiver 38. The filter drumcomprises a rotating, perforated cylinder carrying a coat of uniformthickness of a porous material such as diatomaceous earth. A finediatomaceous silica with an average particle diameter of about 4 micronsis the preferred porous material for forming the precoat on the filter.Materials of coarser particle size are suitable but not preferred. A oneto two inch precoat should be used when fine precoat is used while aprecoat of about four inches in thickness should be applied when theprecoat material has an average particle size of about 6 microns orgreater. A recycle line 41 can be provided from the feed vessel 33 backto the flocculated feed in line 32 whenever a filter aid is utilized inthe process. This recirculation loop serves to maintain the filter aidsuspended in the water. In the filtering zone, the filter drum 34 isrotated so that its filtering surface covered with the filter coat movesthrough the pondwater contained in the feed vessel 33.

Water is drawn from the flocculated middlings through the filter mediumby vacuum into the drum 34 while the flocculated solids contained in themiddlings are retained on the precoat. As the solids are deposited, theyare shaved off together with a thin layer of precoat by means of theshaving blade 35 and are discarded 36. A clarified water is recoveredfrom the drum interior and is passed via line 37 to the filtratereceiver 38 where the water (entrained in the vapor stream) isdisengaged. Vapor is discaarded via line 39 by a vacuum pump 42. Thetreated water can be introduced via line 40 by a filtrate pump 43 intothe system as all or a portion of the water in line 9 to the sump asshown or can be alternatively introduced into the system via lines 2 or10 or as a screen wash or at any desired point of introduction into theprocess.

Although the invention has been described supra with reference to thetreatment of pondwater from the hot water process effluent discharge itshould be pointed out that the invention can be practiced on any waterstream from the separation cell. For example, referring again to thedrawing, the bitumen-lean middlings line 19 from the flotation scavengerzone 16 can be directly treated by the invention to make these middlingssuitable fer recycle back into the process. Also the middlings in line10 can be treated before recycle into sump 8 for dilution of the tarsands pulp.

EXAMPLES The following runs were made on a continuous rotary drum vacuumfilter. Pondwater was first introduced to a flocculating tank where itwas gently agitated and flocculated by the addition of H 50 to pH Inruns using a filter aid the aid was added to the pondwater in theflocculating tank. The pondwater was then pumped by means of a slurrypump to the feed tank of the filter.

Filter drum size was 20 inches diameter by 12 inches (5.124 squarefeet). Submerged drum area was varied by controlling the slurry level inthe feed tank. Submergence time of a given point on the drum wascontrolled by the slurry level and drum revolutions per minute. Rate ofadvance of the filter doctor blade was controlled independent of drumrevolutions per minute by a separate drive. Precoat depth was about oneinch for all runs. Vacuum was 20 inches mercury for all runs. A coarseweave polypropylene fabric was used for all runs to support the precoatiayer. Feed for all runs was hot water process pondwater.

Table I gives the conditions for each run and Table IE shows theresults. The runs in the tabies generally illustrate that vacuumprecoate filtration can be used to produce a clarified water useful forrecycle back into the hot water process for separating bitumen from tarsands. The runs aiso show that a precoat with a particle diameter ofabout 4 microns or less is effective in increasing filter cake porositythereby increasing filter rate. Runs to show that drainage time haslittle significant effect on cake percent water and within tested limitsdoes not limit filter performance. All runs generally show thatincreased submergence time decreases average filtrate rate. Forinstance, the run series 31, 32, 27, 28, l3, 14, 25, 26, 29, 30 and theseries 46, 47, 50, 51, 35 to 40, 48, 49, 52, 53, 83 to 86, 56, 57, 54,55, 66 to 69, 62 to 65, 61, 58, 59 illustrate this point. Runs 5, 6, 9,10, 33, 34 and 70 to 82 show that diatomaceous earth with an averageparticle diameter of less than about 6 microns as a filter aid does givesome effective increase in filtrate rate= TABLE L-FILTRATION TESTRESULTS Operating conditions Filter Aid Pi'ecoat Drum Feed, weightpercent mineral Pounds Type per gallon Type Filter- Col 1 do D. D. 0.'0. l. 1. D. 2. 3. 3.

None Filteir-Cel l Knife advance, inches per revolution Fraction of areasubmerged Drainage time, seconds Revolutions per minute water; passingsaid mixture into a separation zone to form an upper bitumen frothlayer, a middlings layer comprising water, finely divided mineral andbitumen, and a sand tailings layer, and separately removing said bitumenfroth layer, middlings layer and tailings layer, and said process forclarifying water discharged from said hot water process comprises:

(a) flocculating finely divided mineral in at least a portion of saidmiddlings; and

(b) vacuum precoat filtering said portion to remove flocculated mineralto produce a water filtrate substantially reduced in mineral content andsuitable for recycle back into the hot water process as at least aportion of the water utilized to form said mixture of tar sands andwater.

3. The process of claim 2 in which the flocculating step comprisesadding a flocculating agent to fiocculate said mineral component.

4. The process of claim 2 in which the flocculating step comprisesadjusting the pH of said middlings portion below about 7.5.

5. The process of claim 2 in which the flocculating step comprisesadjusting the pH of said middlings portion to above about 9.0.

6. The process of claim 2 in which said water substantially reduced inmineral content is recycled back into the hot water process as at leasta portion of the water utilized to form said mixture of tar sands andwater.

7. The process of claim 2 in which a filter aid comprising diatomaceoussilica with an average particle diameter of less than about 6 microns isadded prior to said vacuum precoat filtering step.

8. The process of claim 2 in which a diatomaceous silica is used as aprecoat in said vacuum precoat filtration step.

9. The process of claim 7 in which a portion of said water filtrate isrecycled to said middlings at the point of addition of said filter aid.

10. In a hot water process for separating bitumen from bituminous tarsands which comprises:

(a) forming a pulp of said bituminous sands with a minor amount of waterin a pulping zone;

(b) removing said pulp therefrom and mixing the same with hot water anda hereinafter specified recycle stream in a dilution zone;

(c) flushing the mixture with water from the dilution zone into aseparation zone;

(d) settling the mixture in said separation zone at a temperature in therange of 130210 F. to form an upper bitumen froth layer, a middlingslayer comprising water, clay and bitumen and a sand tailings layer;

(e) separately removing said bitumen froth layer and said sand tailingslayer;

(f) removing a stream of middlings layer from said separation zone andpassing it to said dilution zone as the aforesaid recycle stream;

(g) passing a second stream of middlings layer to a separate recoveryzone and therein subjecting it to air flotation to recover an additionalamount of bitumen;

(h) regulating the amount of water incorporated with sair bituminoussands and the rate in Step (g) of passage of said second stream to saidseparate recovery zone so as to regulate and maintain the viscosity ofsaid middlings layer within the range of 0.4 to 5.7 centipoises; and

(i) removing fro msaid separate recovery zone middlings material ofdepleted bitumen content comprising clay dispersed in water; theimprovement which comprises:

(j) flocculating clay in at least a portion of said middlings ofdepleted bitumen content;

(k) vacuum precoat filtering said portion to remove fiocculated clay toproduce a water filtrate substantially reduced in mineral content; and

(l) recycling said portion to Step (c) as at least a portion of thewater used to flush said mixture to said separation zone.

11. The process of claim 10 in which Step (1) comprises recycling saidportion to Step (a) as at least a portion of the water used to form saidpulp.

12. The process of claim 10 in which the flocculating Step (j) comprisesadding a flocculating agent to flocculate said clay component.

13. The process of claim 10 in which the flocculating Step (j) comprisesthe pH of said middlings portion to below about 7.5.

14. The process of claim 10 in which the flocculating Step (j) comprisesadjusting the pH of said middlings portion to above about 9.5.

15. The process of claim 10 in which a filter aid comprises diatomaceoussilica with an average particle diameter of less than about 6 microns isadded prior to said vacuum precoat filtering step.

16. The process of claim 10 in which a diatomaceous silica is used as aprecoat in said vacuum precoat filtra tion step.

17. The process of claim 15 in which a portion of said water filtrate isrecycled to said middlings at the point of addition of said filter aid.

References Cited UNITED STATES PATENTS 1,830,962 11/1931 Read et al.21075 X 2,692,229 10/1954 Heise et al. 210X 2,715,466 8/1955 Esposito210-75 3,392,833 7/1968 Baillie 210- JAMES L. DECESARE, Primary ExaminerUS. Cl. X.R. 21060, 75, 196

